Central Neurophysiology of Vision pdf - The Nervous System

signals from the medial half of the retina of the opposite eye. The respective

from the lateral half of the ipsilateral retina, whereas layers I, IV, and VI receive

of six nuclear layers. Layers II, III, and V (from ventral to dorsal) receive signals

are kept apart in the dorsal lateral geniculate nucleus. This nucleus is composed

responding points on the two retinas. However, the signals from the two eyes

chiasm are derived from one eye and half from the other eye, representing cor-

). This relay function is so accurate that

geniculate nucleus serves two principal functions: First, it relays visual infor-

as shown in Figure 51–1. The dorsal lateral

lateral geniculate body,

geniculate nucleus,

The optic nerve fibers of the new visual system terminate in the

of the Thalamus

Function of the Dorsal Lateral Geniculate Nucleus

mammals.

primitive animals, even visual form is detected by the older system, using the

aspects of visual form, colors, and other conscious vision. Conversely, in many

human beings, the new system is responsible for perception of virtually all

mission of visual signals into the visual cortex located in the occipital lobes. In

Thus, the visual pathways can be divided roughly into an

to help control some of the body’s behavioral functions.

of the thalamus and surrounding basal regions of the brain, presumably

tional movements of the two eyes; and (4) into the

superior colliculus,

the pupillary light reflex; (3) into the

to elicit

body with night and day; (2) into the

suprachiasmatic nucleus of the hypothalamus,

Visual fibers also pass to several older areas of the brain: (1) from the optic

of the medial occipital lobe.

there,

The fibers of each optic tract

optic tracts.

opposite sides, where they join the fibers from the

optic chiasm,

nerves.

The visual

Figure 51–1 shows the principal visual pathways

Visual Pathways

Neurophysiology of Vision

The Eye: III. Central

640

from the two retinas to the visual cortex.
nerve signals leave the retinas through the optic

At the

the optic nerve fibers

from the nasal halves of the retinas cross to the

opposite temporal retinas to form the
then synapse in the dorsal lateral geniculate nucleus of the thalamus, and from

geniculocalcarine fibers pass by way of the optic radiation (also called the

geniculocalcarine tract) to the primary visual cortex in the calcarine fissure area

tracts to the

presumably to

control circadian rhythms that synchronize various physiologic changes of the

pretectal nuclei in the midbrain,

reflex movements of the eyes to focus on objects of importance and to activate

to control rapid direc-

ventral lateral geniculate

nucleus

old system

to the midbrain and base of the forebrain and a new system for direct trans-

superior colliculus in the same manner that the visual cortex is used in

dorsal lateral

located at the dorsal end of the thalamus and also called

simply the

mation from the optic tract to the visual cortex by way of the optic radiation
(also called the geniculocalcarine tract
there is exact point-to-point transmission with a high degree of spatial fidelity
all the way from the retina to the visual cortex.

It will be recalled that half the fibers in each optic tract after passing the optic

retinal areas of the two eyes connect with neurons that are superimposed over

Figure 51–2) lies in the

The primary visual cortex (see

Primary Visual Cortex.

secondary visual areas.

systems, the visual cortex is divided into a

primarily on the medial aspect of the occipital lobes.

Figures 51–2 and 51–3 show the

of the Visual Cortex

Organization and Function

rather than at high velocity.

tion, but at only a moderate velocity of conduction

These neurons receive their input almost entirely from

large numbers of small to medium-sized neurons.

parvocellular layers

spread widely in the retina. (2) Layers III through VI

are not many Y ganglion cells, and their dendrites

transmitting only black-and-white information. Also,

the visual cortex. However, this system is color blind,

the large type Y retinal ganglion cells. This magnocel-

neurons. These receive their input almost entirely from

magnocellular layers

divided in another way: (1) Layers I and II are called

Finally, the dorsal lateral geniculate nucleus is

light the visual information that is allowed to pass.

portions of the dorsal lateral geniculate nucleus. It is

stimulated, can turn off transmission through selected

Both of these are inhibitory and, when

geniculate nucleus, and (2)

sources: (1)

much of the signal is allowed to pass to the cortex. The

signals to the visual cortex—that is, to control how

geniculate nucleus is to “gate” the transmission of

The second major function of the dorsal lateral

one another in the paired layers, and similar parallel

The Eye: III. Central Neurophysiology of Vision

Chapter 51

641

transmission is preserved all the way to the visual
cortex.

nucleus receives gating control signals from two major

corticofugal fibers returning in a backward

direction from the primary visual cortex to the lateral

reticular areas of the mes-

encephalon.

assumed that both of these gating circuits help high-

because they contain large

lular system provides a rapidly conducting pathway to

its point-to-point transmission is poor because there

are called

because they contain

the type X retinal ganglion cells that transmit color
and convey accurate point-to-point spatial informa-

visual cortex located

Like the cortical representations of the other sensory

primary

visual cortex and

calcarine fissure area, extend-

ing forward from the occipital pole on the medial

Optic radiation

Optic chiasm

Optic tract

Optic nerve

Left eye

Right eye

Visual cortex

Lateral geniculate body

Superior

colliculus

ified from Polyak SL: The Retina. Chicago: University of Chicago,

Principal visual pathways from the eyes to the visual cortex. (Mod-

Figure 51–1

1941.)

Macula

Secondary

visual areas

Calcarine fissure

Primary

visual cortex

∞

cortex.

calcarine fissure area

Visual cortex in the

Figure 51–2

of the medial occipital

Motor cortex

Somatosensory area I

Form,
3-D position,
motion

Primary
visual
cortex

Secondary
visual
cortex

Visual
detail,

color

ital lobe and the ventral portion of the posterior temporal lobe.

are transmitted mainly into the anteroventral portion of the occip-

superior portions of the occipital lobe and posterior portions of the

dimensional position, and motion are transmitted mainly into the

parietal cortices. Note that the signals representing form, third-

secondary visual areas on the lateral surfaces of the occipital and

Transmission of visual signals from the primary visual cortex into

Figure 51–3

parietal lobe. By contrast, the signals for visual detail and color

much greater distances.

layers V and VI excite neurons that transmit signals

the cortex. Conversely, the signals that pass inward to

signals that pass outward to layers I, II, and III even-

mation at successive stations along the pathway. The

inward along each vertical column unit. This process-

After the optic signals terminate in layer IV, they are

1000 or more neurons.

represents a functional unit. One can roughly calculate

motor and analytical cortical regions). Each column

a diameter of 30 to 50 micrometers. The same vertical

vertical columns of neuronal cells, each column having

The visual

Vertical Neuronal Columns in the Visual Cortex.

surface of the cortex and to deeper layers. It is these

IV, shown to the right in Figure 51–4. From there, these

, the shallowest and deepest portions of layer

ent from the Y signals. They terminate in layers IVa

retina, also terminate in layer IV, but at points differ-

nerve fibers, derived from the X ganglion cells in the

The visual signals from the medium-sized optic

levels.

, and

subdivisions. The rapidly conducted signals from the Y

mainly in layer IV. But this layer, too, is organized into

sensory systems, the geniculocalcarine fibers terminate

shown in Figure 51–4. Also, as is true for the other

the primary visual cortex has six distinct layers, as

Visual Cortex

and so forth—up to more than a dozen areas. The

ary visual areas have specific designations—V-3, V-4,

or simply V-2. The other, more distant second-

next. Therefore, Brodmann’s area 18 is called

(see Figure 51–3), which is where

Brodmann’s area 18

instance, on all sides of the primary visual cortex is

ted to these areas for analysis of visual meanings. For

shown in Figure 51–3. Secondary signals are transmit-

lateral surfaces of the occipital and parietal cortex, as

cortex. Most of these areas also fold outward over the

anterior, superior, and inferior to the primary visual

visual association areas,

areas, also called

The secondary visual

Secondary Visual Areas of the Cortex.

area has a grossly striated appearance.

. Still another name is the

The primary visual cortex is also called

area, the fovea has several hundred times as much rep-

the highest degree of visual acuity. Based on retinal

fovea transmits its signals. The fovea is responsible for

resents the macula. It is to this region that the retinal

and the lower portion inferiorly.

the calcarine fissure on the medial occipital lobe. The

occipital pole, as shown in Figure 51–2, while signals

nus of direct visual signals from the eyes. Signals from

aspect of each occipital cortex. This area is the termi-

The Nervous System: B. The Special Senses

642

Unit X

the macular area of the retina terminate near the

from the more peripheral retina terminate at or in con-
centric half circles anterior to the pole but still along

upper portion of the retina is represented superiorly

Note in the figure the especially large area that rep-

resentation in the primary visual cortex as do the most
peripheral portions of the retina.

visual area

striate cortex because this

lie lateral,

virtually all signals from the primary visual cortex pass

visual

area II,

importance of all these areas is that various aspects
of the visual image are progressively dissected and
analyzed.

Layered Structure of the Primary

Like almost all other portions of the cerebral cortex,

retinal ganglion cells terminate in layer IVc

from there they are relayed vertically both outward
toward the cortical surface and inward toward deeper

and IVc

signals are transmitted vertically both toward the

X ganglion pathways that transmit the accurate point-
to-point type of vision as well as color vision.

cortex is organized structurally into several million

columnar organization is found throughout the cere-
bral cortex for the other senses as well (and also in the

that each of the visual vertical columns has perhaps

further processed as they spread both outward and

ing is believed to decipher separate bits of visual infor-

tually transmit signals for short distances laterally in

“Color Blobs” in the Visual Cortex.

Interspersed among the

primary visual columns as well as among the columns

)

III

Color

“blobs”

LGN

(parvocellular)

Retinal

"X"

ganglion

Very Accurate, Color

LGN

(magnocellular)

Retinal

"Y"

ganglion

Fast, Black and White

(a)

(b)

)

blobs,” which are necessary for detection of color.

color. Note especially the areas of the visual cortex called “color

inate in the parvocellular layers (layers III through VI) of the LGN;

the left side of the figure originate in the magnocellular layers of

Six layers of the primary visual cortex. The connections shown on

Figure 51–4

the lateral geniculate nucleus (LGN) and transmit rapidly chang-
ing black-and-white visual signals. The pathways to the right orig-

they transmit signals that depict accurate spatial detail as well as

The visual cortex detects not only the

areas, the greater the degree of stimulation.

that is, the greater the sharpness of contrast and the

does not occur, and the intensity of stimulation of most

from dark to light or light to dark, mutual inhibition

any border in the visual scene where there is a change

retinal receptors mutually inhibit one another. But at

glion cells as well, because equally stimulated adjacent

rather than with noncontrasting areas. We saw in

in the visual scene,

Thus, the visual signal in the primary visual cortex is

occur along the sharp borders of the visual pattern.

left in Figure 51–5. To the right is shown the spatial

place on the wall a large solid cross, as shown to the

primary visual cortex detect? To answer this, let us now

fore, the question must be asked, What does the

the illumination of the wall is bright or weak. There-

visual cortex will be stimulated, regardless of whether

at a blank wall, only a few neurons in the primary

of the Visual Image

Neuronal Patterns of

it means.

mining detailed colors of objects, and deciphering

reading, determining the texture of surfaces, deter-

concerned with such visual feats as recognizing letters,

dissect out color as well. Therefore, this pathway is

detail. Separate portions of this pathway specifically

inferior, ventral,

Figure 51–3, passing from the primary visual cortex

The red arrows in

2. Analysis of Visual Detail and Color.

color.

of the retinal Y ganglion cells, transmitting rapid

pathway are mainly from the large Y optic nerve fibers

dimensional aspects of somatosensory signals. The

cortex, the signals overlap with signals from the pos-

cortex, the signals flow generally into the

whether it is moving. After leaving the primary visual

as motion in the scene. In other words, this pathway

in the space around the body. This pathway also ana-

demonstrated in Figure 51–3 by the black arrows, ana-

One of the analytical pathways,

pathways in the secondary visual areas.

cortex, the visual information is analyzed in two major

Figure 51–3 shows that after leaving the primary visual

(2) The Accurate Color Pathway

Visual Information—(1) The Fast

Two Major Pathways for Analysis of

stereopsis.

fuse with each other (be brought into “register”). The

turn, the deciphered information is used to adjust the

ding points from the two retinas fit with each other. In

two visual images from the two separate eyes are “in

nating with signals from the second eye. This cortical

eye enter the columns of every other stripe, alter-

stripe about 0.5 millimeter wide; the signals from one

is interlaced with stripes of neuronal columns, each

layer IV of the primary visual cortex. In fact, layer IV

the lateral geniculate nucleus. These signals still

eyes are relayed through separate neuronal layers in

Interaction of Visual Signals from the Two Separate Eyes.

deciphering color.

activated specifically by color signals. Therefore, it is

They receive

color blobs.

The Eye: III. Central Neurophysiology of Vision

Chapter 51

643

of some of the secondary visual areas are special
column-like areas called
lateral signals from adjacent visual columns and are

presumed that these blobs are the primary areas for

Recall that the visual signals from the two separate

remain separated from each other when they arrive in

area deciphers whether the respective areas of the

directional gaze of the separate eyes so that they will

information observed about degree of register of
images from the two eyes also allows a person to dis-
tinguish the distance of objects by the mechanism of

“Position” and “Motion” Pathway;

1. Analysis of Third-Dimensional Position, Gross Form, and
Motion of Objects.

lyzes the third-dimensional positions of visual objects

lyzes the gross physical form of the visual scene as well

tells where every object is during each instant and

posterior

midtemporal area and upward into the broad occipi-
toparietal cortex. At the anterior border of the parietal

terior somatic association areas that analyze three-

signals transmitted in this position-form-motion

signals but depicting only black and white with no

into secondary visual areas of the

and

medial regions of the occipital and temporal cortex,
show the principal pathway for analysis of visual

from all this information what the object is and what

Stimulation During Analysis

Analysis of Contrasts in the Visual Image.

If a person looks

pattern of the most excited neurons in the visual
cortex. Note that the areas of maximum excitation

concerned mainly with contrasts

Chapter 50 that this is true of most of the retinal gan-

neurons is proportional to the gradient of contrast—

greater the intensity difference between light and dark

Visual Cortex Also Detects Orientation of Lines and Borders—
“Simple” Cells.

existence of lines and borders in the different areas of
the retinal image but also the direction of orientation

Retinal image

Cortical stimulation

to a retinal image of a dark cross.

Pattern of excitation that occurs in the visual cortex in response

Figure 51–5

or excessive use of tobacco.

retina, or from toxic conditions such as lead poisoning

pressure in the eyeball), from allergic reactions in the

than the optic disc area. Such blind spots are called

Occasionally, blind

in the figure.

degrees lateral to the central point of vision, as shown

try charts, a

left eye is plotted as shown in Figure 51–6. In all perime-

seen and when it cannot. Thus, the field of vision for the

and forth in all areas of the field of vision, and the

Then a small dot of light or a small object is moved back

looking toward a central spot directly in front of the eye.

This is done by having the

perimetry.

retina, one charts the field of vision for each eye by a

To diagnose blindness in specific portions of the

given instant. The area seen to the nasal side is called

is the visual area seen by an eye at a

The

Fields of Vision; Perimetry

head, and avoidance. This vision is believed to be sub-

These reactions include turning the eyes, turning the

scene, or, rarely, even to some gross patterns of vision.

to changes in light intensity, to movement in the visual

“blind” people can still, at times, react subconsciously

However, psychological studies demonstrate that such

being causes loss of conscious vision, that is, blindness.

Visual Cortex

Effect of Removing the Primary

detected by complex and hypercomplex cells.

simple cells, whereas more complex contrasts are

colors,” excite specific neuronal cells. It is presumed

on the fact that contrasting colors, called “opponent

The mechanism of color contrast analysis depends

has changed the color entering the eyes.

the color of the “white” changes with the light, and

that is, when the color of an illuminating light changes,

sible for the phenomenon called “color constancy”;

white area within the visual scene. In fact, this con-

area. All these colors can also be contrasted against a

area against a red area, or a green area against a yellow

red area is often contrasted against a green area, a blue

detected: by means of color contrast. For instance, a

Detection of Color

ical pathway of the visual cortex, progressively more

visual scene. Thus, as one goes farther into the analyt-

that have other characteristics. That is, these neurons

cific lengths, by specific angulated shapes, or by images

areas, are stimulated only by lines or borders of spe-

columns, as well as neurons in some secondary visual

complex cells.

be stimulated if the line has the same direction. These

or vertically in the field, the same few neurons will still

same direction but are not position-specific. That is,

the visual signal progresses farther away from layer IV,

ally or Vertically in the Visual Field—“Complex” Cells.

They are found mainly in layer IV of the primary

simple cells.

set of cells. These neuronal cells are called

tion of a line, specific neuronal cells are stimulated. A

there is a contrast edge. Thus, for each such orienta-

horizontal or lies at some degree of inclination. This is

of each line or border—that is, whether it is vertical or

The Nervous System: B. The Special Senses

644

Unit X

believed to result from linear organizations of mutu-
ally inhibiting cells that excite second-order neurons
when inhibition occurs all along a line of cells where

line oriented in a different direction excites a different

visual cortex.

Detection of Line Orientation When a Line Is Displaced Later-

some neurons respond to lines that are oriented in the

even if a line is displaced moderate distances laterally

cells are called

Detection of Lines of Specific Lengths, Angles, or Other Shapes.

Some neurons in the outer layers of the primary visual

detect still higher orders of information from the

characteristics of each visual scene are deciphered.

Color is detected in much the same way that lines are

trasting against white is believed to be mainly respon-

appropriate computation in the brain allows red to be
interpreted as red even though the illuminating light

that the initial details of color contrast are detected by

Removal of the primary visual cortex in the human

served by neuronal pathways that pass from the optic
tracts mainly into the superior colliculi and other por-
tions of the older visual system.

field of vision

the nasal field of vision, and the area seen to the lateral
side is called the temporal field of vision.

process called
subject look with one eye closed and the other eye

subject indicates when the spot of light or object can be

blind spot caused by lack of rods and cones

in the retina over the optic disc is found about 15

Abnormalities in the Fields of Vision.

spots are found in portions of the field of vision other

sco-

tomata; they frequently are caused by damage to the
optic nerve resulting from glaucoma (too much fluid

90
80
70
60
50

345

330

315

300

285

255

270

240

225

210

195

180

165

150

135

Left

Optic

disc

Right

120

105

Perimetry chart, showing the field of vision for the left eye.

Figure 51–6

tical eye fields automatically “lock” the eyes on a given

To summarize, posterior “involuntary” occipital cor-

or may become totally unable to do so.

bilaterally in an animal, the animal has difficulty

visual cortex. When this fixation area is destroyed

eyes to “lock” on the object of attention once it is

Conversely, the fixation mechanism that causes the

a short time, which then allows the eyes to be moved.

and move them to another point. It is usually neces-

person to “unlock” the eyes from one point of fixation

51–8. Bilateral dysfunction or destruction of these

tical regions of the frontal lobes, as shown in Figure

The voluntary fixation movements are controlled by

involuntary fixation mechanism.

eyes firmly on the object once it has been found; this

The second is an involuntary mechanism that holds the

voluntary fixation mechanism.

vision; this is called the

find the object on which he or she wants to fix the

controlled by two neuronal mechanisms. The first of

portion of the field of vision. Fixation movements are

are those that cause the eyes to “fix” on a discrete

Fixation Movements of the Eyes

mitted from the body’s equilibrium control centers in

of the oculomotor nerves. Strong signals are also trans-

the pretectal and the superior colliculus areas, the ocu-

superior colliculus areas of the brain stem. From both

oculomotor apparatus, showing spread of signals from

Figure 51–8 demonstrates cortical control of the

pair relaxes while the other contracts.

fasciculus.

peripheral nerves to the ocular muscles. Shown, too,

also shows brain stem nuclei for the third, fourth, and

Figure 51–7

downward. The oblique muscles function mainly to

to move the eyes from side to side. The superior and

The medial and lateral recti contract

inferior obliques.

Figure 51–7: (1) the

are controlled by three pairs of muscles, shown in

The eye movements

To make full use of the visual abilities of the eyes,

Their Control

opposite side of the head. This condition is known as

lesion; as a result, neither eye can see objects to the

the optical system of the eye; this condition is called

retina is blinded, which means that the person is blind

opposite optic tract. Therefore, the nasal half of each

optic chiasm

blindness of the affected eye.

Vision.

Effect of Lesions in the Optic Pathway on the Fields of

central areas.

pigment deposits in the degenerated areas. Retinitis pig-

tions of the retina degenerate, and excessive melanin

In this disease, por-

The Eye: III. Central Neurophysiology of Vision

Chapter 51

645

Still another condition that can be diagnosed by

perimetry is retinitis pigmentosa.

mentosa usually causes blindness in the peripheral field
of vision first and then gradually encroaches on the

Destruction of an entire optic nerve causes

Destruction of the

prevents the crossing

of impulses from the nasal half of each retina to the

in the temporal field of vision for each eye because the
image of the field of vision is inverted on the retina by

bitemporal hemianopsia. Such lesions frequently result
from tumors of the pituitary gland pressing upward
from the sella turcica on the bottom of the optic chiasm.

Interruption of an optic tract denervates the corre-

sponding half of each retina on the same side as the

homonymous hemianopsia.

Eye Movements and

almost equally as important as interpretation of the
visual signals from the eyes is the cerebral control
system for directing the eyes toward the object to be
viewed.

Muscular Control of Eye Movements.

medial and lateral recti, (2) the

superior and inferior recti, and (3) the superior and

inferior recti contract to move the eyes upward or

rotate the eyeballs to keep the visual fields in the
upright position.

Neural Pathways for Control of Eye Movements.

sixth cranial nerves and their connections with the

are interconnections among the brain stem nuclei by
way of the nerve tract called the medial longitudinal

Each of the three sets of muscles to each

eye is reciprocally innervated so that one muscle of the

visual areas in the occipital cortex through occipito-
tectal and occipitocollicular tracts to the pretectal and

lomotor control signals pass to the brain stem nuclei

the brain stem into the oculomotor system (from the
vestibular nuclei by way of the medial longitudinal
fasciculus).

Perhaps the most important movements of the eyes

these allows a person to move the eyes voluntarily to

is called the

a cortical field located bilaterally in the premotor cor-

areas makes it difficult or almost impossible for a

sary to blink the eyes or put a hand over the eyes for

found is controlled by secondary visual areas in the
occipital cortex, located mainly anterior to the primary

keeping its eyes directed toward a given fixation point

Lateral

rectus

Inferior oblique

Medial
longitudinal
fasciculus

Superior oblique

Nuclei

N.III

N.IV

N.VI

Superior rectus

Inferior rectus

Medial rectus

Figure 51–7

Extraocular muscles of the eye and their innervation.

important information. Similar saccades occur when a

the visual scene is not moving past the eyes, but the

cadic movements of the eyes for each line. In this case,

process of reading, a person usually makes several sac-

presses the visual image during saccades, so that the

allocated to the fixation sites. Also, the brain sup-

moving the eyes, with 90 per cent of the time being

The saccades occur so rapidly that no

movements.

saccades,

of two to three jumps per second. The jumps are called

the visual field, jumping from one to the next at a rate

in a car, the eyes fix on one highlight after another in

ually before the eyes, such as when a person is riding

When a visual scene is moving contin-

Saccadic Movement of the Eyes—A Mechanism of Successive

region. This involuntary fixation capability is mostly

strated in Figure 51–9, which shows by the dashed lines

These drifting and flicking motions are demon-

center of the fovea.Thus, an automatic response moves

reaction occurs, producing a flicking movement that

drifts as far as the edge of the fovea, a sudden reflex

to drift slowly across the cones. Each time the spot

the cones, and the drifting movements cause the spot

region of the retina, the tremulous movements cause

When a spot of light has become fixed on the foveal

flicking move-

direction or another, and (3) sudden

slow drift

ocular muscles, (2) a

at a rate of 30 to 80 cycles per second caused

almost imperceptible movements: (1) a

The eyes normally have three types of continuous but

The involuntary locking type of fixation

Mechanism of Involuntary Locking Fixation—Role of the Supe-

cortices.

cortical “voluntary” eye fields located in the frontal

fixation, voluntary signals must be transmitted from

of the image across the retinas. To unlock this visual

The Nervous System: B. The Special Senses

646

Unit X

spot of the visual field and thereby prevent movement

rior Colliculi.

discussed in the previous section results from a nega-
tive feedback mechanism that prevents the object of
attention from leaving the foveal portion of the retina.

continuous

tremor
by successive contractions of the motor units in the

of the eyeballs in one

ments that are controlled by the involuntary fixation
mechanism.

the spot to move back and forth at a rapid rate across

moves the spot away from this edge back toward the

the image back toward the central point of vision.

the slow drifting across the fovea and by the solid lines
the flicks that keep the image from leaving the foveal

lost when the superior colliculi are destroyed.

Fixation Points.

and the movements are called opticokinetic

more than 10 per cent of the total time is spent in

person is not conscious of the movements from point
to point.

Saccadic Movements During Reading.

During the

eyes are trained to move by means of several succes-
sive saccades across the visual scene to extract the

Visual
association
areas

Primary
visual cortex

Occipitotectal and
occipitocollicular tracts

Pretectal nuclei

Visceral nucleus III nerve

Superior colliculus

Oculomotor nucleus

Frontotectal tract

Voluntary fixation

area

Involuntary

fixation

area

III nerve

IV nerve

VI nerve

Medial longitudinal

Trochlear nucleus

Abducens nucleus

Vestibular nuclei

Neural pathways for control of

Figure 51–8

conjugate movement of the eyes.

ter at the same time. Furthermore, the nearer the

each other, it is still impossible for all corresponding

ity. Therefore, even when the two eyes are fused with

side, and the closer the object, the greater the dispar-

object, and the left eye a little more of the left-hand

two retinas are not exactly the same. That is, the right

eyes are more than 2 inches apart, the images on the

In Chapter 49, it is pointed out that because the two

Distances of Visual Objects

Neural Mechanism of Stereopsis for Judging

visual cortex disappears.

excitation of the specific “interference” neurons in the

responding points of the two retinas are in register,

eyes so that fusion can be re-established. Once the cor-

“fused.” This excitation presumably provides the

images are not “in register”—that is, are not precisely

neurons in the visual cortex. Interactions occur

body, and these signals in turn are relayed to parallel

The visual cortex plays an important role in fusion. It

other on “corresponding points” of the two retinas.

To make the visual perceptions more meaningful, the

the Two Eyes

of the Visual Images from

visual, auditory, or somatic.

respect to external disturbances, whether they are

global role in orienting the eyes, head, and body with

liculi are intact. Therefore, the superior colliculi play a

of the eyes, head, and body, but only if the superior col-

stroking of the side of the body, cause similar turning

visual disturbances, such as strong sounds or even

the direction of the disturbance. Other types of non-

visual disturbance, signals are relayed from the supe-

visual pathway, but their function is unclear.)

type W optic nerve fibers. These represent the oldest

the other going to the superior colliculi. (The superior

fibers, with one branch going to the visual cortex and

ments are branches from the rapidly conducting Y

The optic nerve fibers from the eyes to the colliculi

from the body and acoustic signals from the ears.

directional movement of the eyes, the superior colli-

motor nuclei to turn the eyes. To help in this

racy. Even so, the principal direction of a flash of light

in the primary visual cortex, although with less accu-

have also been destroyed. To support this function, the

direction. This does not occur if the superior colliculi

Even after the visual cortex has been destroyed, a

Visual Disturbance

for Turning the Eyes and Head Toward a

Superior Colliculi Are Mainly Responsible

pursuit system for controlling eye movements.

almost exactly. This represents a high degree of auto-

seconds, the eyes develop progressively smoother

ment as that of the object. Then, after another few

so, the eyes begin to jump by means of saccades in

be unable to fixate on it. However, after a second or

rate of several times per second, the eyes at first may

course of movement for the eyes. For instance, if an

pursuit movement.

can also remain fixed on a moving object, which is

The eyes

painting to another, and so forth.

occur in upward, sideways, downward, and angulated

person observes a painting, except that the saccades

The Eye: III. Central Neurophysiology of Vision

Chapter 51

647

directions one after another from one highlight of the

Fixation on Moving Objects—“Pursuit Movement.”

called

A highly developed cortical

mechanism automatically detects the course of move-
ment of an object and then rapidly develops a similar

object is moving up and down in a wavelike form at a

approximately the same wavelike pattern of move-

movements and finally follow the wave movement

matic subconscious computational ability by the

sudden visual disturbance in a lateral area of the visual
field often causes immediate turning of the eyes in that

various points of the retina are represented topo-
graphically in the superior colliculi in the same way as

in a peripheral retinal field is mapped by the colliculi,
and secondary signals are transmitted to the oculo-

culi also have topological maps of somatic sensations

that are responsible for these rapid turning move-

colliculi and other regions of the brain stem are also
strongly supplied with visual signals transmitted in

In addition to causing the eyes to turn toward a

rior colliculi through the medial longitudinal fascicu-
lus to other levels of the brain stem to cause turning
of the whole head and even of the whole body toward

“Fusion”

visual images in the two eyes normally fuse with each

was pointed out earlier in the chapter that correspon-
ding points of the two retinas transmit visual signals to
different neuronal layers of the lateral geniculate

between these cortical neurons to cause interference
excitation in specific neurons when the two visual

signal that is transmitted to the oculomotor apparatus
to cause convergence or divergence or rotation of the

eye sees a little more of the right-hand side of the

points in the two visual images to be exactly in regis-

Voluntary
movement to
fixation site

American Physiological Society, 1960.)

VE (eds): Handbook of Physiology. vol. 2, sec. 1. Washington, DC,

Central control of the eye movements. In Field J, Magoun HW, Hall

resent sudden flicking movements.) (Modified from Whitteridge D:

lines represent slow drifting movements, and the solid lines rep-

ing” eye movements that move the spot back toward the center

Figure 51–9

Movements of a spot of light on the fovea, showing sudden “flick-

of the fovea whenever it drifts to the foveal edge. (The dashed

until they reach the eye. There, the sympathetic fibers

with postganglionic neurons. Postganglionic sympa-

segment of the spinal cord. From there, sympathetic

The sympathetic innervation of the eye originates in

into the eyeball. These nerves excite (1)

pathetic neurons, which in turn send fibers through

which lies immediately behind the eye. There, the pre-

third nerve

Edinger-Westphal nucleus

shown in Figure 51–11. The parasympathetic pregan-

both parasympathetic and sympathetic nerve fibers, as

The eye is innervated by

Autonomic Nerves to the Eyes.

Pupillary Aperture

Autonomic Control of

normally receive signals from the repressed eye.

fact, even anatomically, the numbers of neuronal con-

nervous system synaptic connections from the eyes. In

young children. This demonstrates that visual acuity is

becomes blinded, vision in the repressed eye can

remaining 20/400 or less. If the dominant eye then

the repressed eye develops only slightly, sometimes

and is never used for precise vision. The visual acuity of

is used all the time, and the other eye becomes repressed

the object of attention. In other patients, one eye alone

patients with strabismus, the eyes alternate in fixing on

Suppression of the Visual Image from a Repressed Eye.

never fuse.

neuronal control pathways themselves, so that the eyes

movements of the eyes become abnormally “set” in the

never simultaneously. Soon the patterns of conjugate

the other fails do so, or they both fixate satisfactorily but

same object, one of the eyes fixates satisfactorily while

young child’s early efforts to fixate the two eyes on the

fusion mechanism of the visual system. That is, in a

Strabismus is often caused by abnormal “set” of the

of the different types of strabismus often occur.

tical strabismus.

torsional strabismus,

izontal strabismus,

types of strabismus are shown in Figure 51–10: (1)

dinates: horizontal, vertical, or rotational. The basic

cross-eye,

Strabismus, also called

for stereopsis.

are excited by nonregister or register. This phenome-

in register for objects 25 meters away. Thus, the dis-

objects 2 meters away; still another set of pathways is

each side of the central pathway. Therefore, some optic

The neuronal cellular mechanism for stereopsis is

stereopsis,

object is to the eyes, the less the degree of register. This

The Nervous System: B. The Special Senses

648

Unit X

degree of nonregister provides the neural mechanism
for

an important mechanism for judging

the distances of visual objects up to about 200 feet
(60 meters).

based on the fact that some of the fiber pathways from
the retinas to the visual cortex stray 1 to 2 degrees on

pathways from the two eyes are exactly in register for

tance is determined by which set or sets of pathways

non is called depth perception, which is another name

Strabismus

squint or

means lack

of fusion of the eyes in one or more of the visual coor-

hor-

(2)

and (3) ver-

Combinations of two or even all three

In a few

develop only to a slight extent in adults but far more in

highly dependent on proper development of central

nections diminish in the visual cortex areas that would

Accommodation and

glionic fibers arise in the
(the visceral nucleus portion of the third cranial nerve)
and then pass in the

to the ciliary ganglion,

ganglionic fibers synapse with postganglionic parasym-

ciliary nerves
the ciliary muscle that controls focusing of the eye lens
and (2) the sphincter of the iris that constricts the
pupil.

the intermediolateral horn cells of the first thoracic

fibers enter the sympathetic chain and pass upward to
the superior cervical ganglion, where they synapse

thetic fibers from these then spread along the surfaces
of the carotid artery and successively smaller arteries

Horizontal

strabismus

Torsional

strabismus

Vertical

strabismus

Figure 51–10

Basic types of strabismus.

Pretectal

region

Edinger-

Westphal

nucleus

Upper thoracic segments

of spinal cord

Ciliary

ganglion

N.III

N.II

Pons

Carotid plexus

Superior cervical
sympathetic
ganglion

Cervical
sympathetic
trunk

the light reflex. (Modified from Ranson SW, Clark SL: Anatomy of

Autonomic innervation of the eye, showing also the reflex arc of

Figure 51–11

the Nervous System: Its Development and Function, 10th ed.
Philadelphia: WB Saunders, 1959.)

the same time. This is called the

near object, the signals that cause accommodation of

other pathway. For instance, when the eyes fixate on a

Edinger-Westphal nucleus is stimulated through some

Yet the pupils can constrict a little more if the

pupils to remain mostly constricted, in addition to their

lost, the nucleus becomes chronically active, causing the

of the inhibitory type. When their inhibitory effect is

tectal area to the Edinger-Westphal nucleus are mostly

The final nerve fibers in the pathway through the pre-

nerves.

pretectal region of the brain stem, although it can result

and so forth.The block usually occurs in the

encephalitis,

central nervous system syphilis, alcoholism,

the pupillary reflexes. Such blocks frequently occur as a

the Edinger-Westphal nucleus, thus sometimes blocking

Pupillary Reflexes or Reactions in Central Nervous System

change in the amount of light entering the eye.

is about 30 to 1—that is, up to as much as 30 times

of pupillary diameter, the range of light and dark adap-

and 8 millimeters on the large side. Therefore, because

as explained in Chapter 50. The limits of pupillary

adapt extremely rapidly to changing light conditions,

The function of the light reflex is to help the eye

ited, which results in dilation of the pupil.

iris. Conversely, in darkness, the reflex becomes inhib-

and, finally, back through

Edinger-Westphal nucleus

tal nuclei. From here, secondary impulses pass to the

When light impinges on the retina, a few of the result-

strated by the upper two black traces in Figure 51–11.

The neuronal pathway for this reflex is demon-

the pupils constrict, a reaction called the

When light is shone into the eyes,

Pupillary Light Reflex.

mydriasis.

Conversely,

miosis.

pupillary aperture; this is called

the pupillary sphincter muscle, thereby decreasing the

Control of Pupillary Diameter

parasympathetic nerve fibers to the eyes.

Edinger-Westphal nucleus, and finally by way of

pretectal area in the brain stem, then through the

Brodmann’s cortical areas 18 and 19 and transmission

ments of the eyes, with analysis of the visual signals in

The brain cortical areas that control accommoda-

provide appropriate focus.

direction. This could give a rapid clue as to which

The visual image becomes clearer when the

the time at a frequency up to twice per second.

4. It has been found that

is different from the clarity of focus on the edges.

retina, the clarity of focus in the depth of the fovea

Because the fovea lies in a hollowed-out depression

strengthen the lens of the eye.

must converge. The neural mechanisms for

2. When the eyes fixate on a near object, the eyes

lens stronger or weaker.

focus, and this clue relays information to the

more than red rays. The eyes appear to be able to

That is, red light rays focus slightly posteriorly to

a fraction of a second. Second, different types of

fixation point, the lens changes its strength in the

First, when the eyes suddenly change distance of the

some of the known features are the following.

this rapid and accurate focusing of the eye is unclear,

focus on a near object, the lens usually accommodates

degree of visual acuity. When the eyes have been

causes decreased power. How does a person adjust

of the lens, as explained in Chapter 49, and relaxation

muscle. Contraction causes increased refractive power

tial for a high degree of visual acuity. Accommodation

The accommodation mechanism—that is, the mecha-

Control of Accommodation

Horner’s syndrome.

pupil) as well as several extraocular muscles of the eye,

The Eye: III. Central Neurophysiology of Vision

Chapter 51

649

innervate the radial fibers of the iris (which open the

which are discussed subsequently in relation to

(Focusing the Eyes)

nism that focuses the lens system of the eye—is essen-

results from contraction or relaxation of the eye ciliary

accommodation to keep the eyes in focus all the time?

Accommodation of the lens is regulated by a nega-

tive feedback mechanism that automatically adjusts
the refractive power of the lens to achieve the highest

focused on some far object and must then suddenly

for best acuity of vision within less than 1 second.
Although the precise control mechanism that causes

proper direction to achieve a new state of focus within

clues help to change the lens strength in the proper
direction:
1. Chromatic aberration appears to be important.

blue light rays because the lens bends blue rays

detect which of these two types of rays is in better

accommodation mechanism whether to make the

convergence cause a simultaneous signal to

that is slightly deeper than the remainder of the

has been suggested that this also gives clues about
which way the strength of the lens needs to be
changed.

the degree of

accommodation of the lens oscillates slightly all

oscillation of the lens strength is changing in
the appropriate direction and becomes poorer
when the lens strength is changing in the wrong

way the strength of the lens needs to change to

tion closely parallel those that control fixation move-

of motor signals to the ciliary muscle through the

Stimulation of the parasympathetic nerves also excites

stimulation of the sympathetic nerves excites the
radial fibers of the iris and causes pupillary dilation,
called

pupillary light

reflex.

ing impulses pass from the optic nerves to the pretec-

parasympathetic nerves to constrict the sphincter of the

diameter are about 1.5 millimeters on the small side

light brightness on the retina increases with the square

tation that can be brought about by the pupillary reflex

Disease.

A few central nervous system diseases damage

nerve transmission of visual signals from the retinas to

result of

from destruction of some small fibers in the optic

failure to respond to light.

the lens and those that cause convergence of the two
eyes cause a mild degree of pupillary constriction at

pupillary reaction to

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affected by Horner’s syndrome.

Fourth, sweating (which requires sympathetic nerve

normally. Third, the blood vessels on the corresponding

Therefore, destruction of the sympathetic nerves makes

superior eyelid and innervated by the sympathetics.

diameter than the pupil of the opposite eye. Second, the

sympathetic nerve fibers to the pupillary dilator muscle,

of the following effects: First, because of interruption of

Horner’s syndrome,

in the cervical sympathetic chain. This causes the clini-

occasionally interrupted. Interruption frequently occurs

The sympathetic nerves to the eye are

Horner’s Syndrome.

syphilis.

The Nervous System: B. The Special Senses

650

Unit X

accommodation. A pupil that fails to respond to light
but does respond to accommodation and is also very
small (an Argyll Robertson pupil) is an important diag-
nostic sign of central nervous system disease—often

cal condition called

which consists

the pupil remains persistently constricted to a smaller

superior eyelid droops because it is normally main-
tained in an open position during waking hours partly
by contraction of smooth muscle fibers embedded in the

it impossible to open the superior eyelid as widely as

side of the face and head become persistently dilated.

signals) cannot occur on the side of the face and head

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